1. Field of the Invention
The present invention relates to a surface emitting laser, and particularly relates to a surface emitting laser of a blue band using a photonic crystal.
2. Description of the Related Art
In recent years, a surface emitting laser which can emit light in a direction vertical to a substrate has received attention.
For instance, a vertical resonator type surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser) is a laser which emits a laser beam in a direction vertical to a substrate, and has an advantage of being easily two-dimensionally arrayed in high density.
Japanese Patent Application Laid-Open No. 2006-165309 proposes a surface emitting laser provided with a photonic crystal structure.
FIG. 10 illustrates a structure of the surface emitting laser using the photonic crystal structure disclosed in Japanese Patent Application Laid-Open No. 2006-165309.
This surface emitting laser has an n-type cladding layer 1004, an active layer 1005, a first p-type cladding layer 1006, a photonic crystal structure 1007, a second p-type cladding layer 1008 and a contact layer 1009 laminated in this order on a substrate 1003, as is illustrated in FIG. 10.
This photonic crystal structure 1007 is formed by providing holes in a semiconductor layer.
This laser adopts a structure of confining light by an optical waveguide which is constituted by the active layer 1005 as the core layer of the waveguide, the n-type cladding layer 1004 and the p-type cladding layers 1006 and 1008.
A periodic change of refractive index in a plane direction originating in the photonic crystal structure causes a feedback function for light, and a resonator having a resonance mode in the in-plane direction is formed due to the feedback function.
This photonic crystal structure also plays a role of a diffraction grating which diffracts a part of a light resonating in the in-plane direction to a vertical direction.
In other words, the resonance direction of the surface emitting laser described in Japanese Patent Application Laid-Open No. 2006-165309 is a horizontal direction with respect to the substrate, and the surface emitting laser emits a part of the resonating light to a vertical direction by a high order diffraction of the photonic crystal structure.
The structure disclosed in Japanese Patent Application Laid-Open No. 2006-165309 needs a thick first p-type cladding layer 1006 and a thick second p-type cladding layer 1008 in order to confine light.
However, such a thick cladding layer causes the increase of electric resistance, consequently causes the increase of voltage and the degradation of power efficiency, and gives an influence on device characteristics.
In addition, the first p-type cladding layer 1006 and the second p-type cladding layer 1008 need to have a lower refractive index than an active layer 1005, and accordingly employ a semiconductor of a ternary solid solution crystal containing Al, for their materials in many cases.
For instance, in Japanese Patent Application Laid-Open No. 2006-165309, Al0.08Ga0.92N is used as the p-type cladding layer 1008.
However, such a ternary solid solution crystal has a problem of showing higher electric resistance than a binary crystal. Then, it is considered to reduce the thickness of the p-type cladding layer, in order to lower the electric resistance.
However, when the thickness of the p-type cladding layer is simply reduced, the distance between an electrode and the active layer is narrowed, which causes a problem that the electrode absorbs the horizontally resonating light. In other words, when the p-type cladding layer is thinned so as to lower the electric resistance, the p-type cladding layer cannot inhibit light from leaking, and the light is absorbed by the electrode, which may reduce emission efficiency.